The plunging pound and stock market speculators are driving up the cost of petrol, according to the AA.

After surging 5p a litre over a month, the price of petrol at the pumps has gone up a further 1p in the last five days, the AA said.

It revealed that the average cost of petrol in the UK is now 138.32p a litre, with diesel having risen 4.78p from its mid-January price to stand at an average of 145.10p.

The latest figures show that petrol has risen 6.24p a litre since early January, adding £3.12 to the cost of refilling a typical 50-litre tank.

The AA said filling up the 70-litre tank of a Ford Mondeo now costs £4.37 more than six weeks ago. A two-car family's monthly petrol cost has risen £13.25 with the current price surge.

It added that drivers have been caught between the pincers of a pound weakened against the dollar and soaring wholesale prices, both due to stock market speculation.

Regionally, Yorkshire and Humberside and the north of England are the cheapest for petrol at the moment at 137.6p a litre, with prices in London and Scotland at an average of 137.8p. Northern Ireland is the most expensive at 138.7p.

Yorkshire and Humberside remains the cheapest region for diesel, averaging 144.2p, while East Anglia, Northern Ireland and south-east England are the most expensive at 145.2p.

AA president Edmund King said: "This latest surge in fuel prices and its impact on spending indicates that UK drivers and families can't take any more. We're no longer talking of the motorist as a cash cow for tax and speculator greed, but a horse slowly but surely being flogged to death.

"This is the third 10p-a-litre wholesale price surge in 11 months, given extra vigour by currency speculators betting against the pound."

Mr King went on: "Given the lashing motoring families and UK businesses are taking from speculator-driven fuel prices, we hope the Chancellor spells out clearly in the forthcoming Budget that he can feel the pressure rocketing fuel price inflation places on families and business, and that he will cancel the September rise if that strain is too great."

The AA also reported that official figures showed January's UK petrol sales fell to the lowest tracked by government in 23 years.

Drivers used 1.465 billion litres of petrol last month, down 14 million on the previous all-time low set in March last year and nearly 100 million below December's consumption of 1.564 billion.

The AA said UK diesel consumption also fell in January, down to 1.923 billion litres for cars, haulage and other uses.

However, this is still higher than the all-time lows of 1.871 billion litres in January 2009 and 1.833 billion in January 2010, when widespread and extended periods of heavy snow cut road use.

Mr King said: "Although we had a disruptive period of snow between January 18 and 25, the impact of the weather was nothing like that suffered during the January of 2010.

"The blame for this latest collapse in petrol sales rests squarely with stock market speculation."